A wireless IC card system has been proposed that employs a card reader to read data stored on an IC card (such as a cash card, a credit card, an ID (identification) card or a memory card) in which a microcomputer and a memory are mounted. In such systems, the exchange of data is not performed in the same manner as it is in a wired IC card system where metal terminals on a card are electrically connected directly to an external input/output device. Instead, antennas are used to exchange data in the form of vibrational energy generated by a high frequency electromagnetic field or by ultrasonic waves. Therefore, the wireless IC card system does not have the problems that are inherent to the wired IC card system, such as contact failures occasioned by the abrasion or the corrosion of metal terminals, or the accumulation of dust and dirt on the terminals, and the electrostatic damage of card components.
Japanese Unexamined Patent Publication No. Hei 2-19989 discloses a system that provides for the mounting on a card of a dipole antenna resonant at a specific frequency, and for an impedance at the antenna to be varied in accordance with the contents of the data to be transmitted to a card reader. Specifically, the resonance of the dipole antenna is employed as a reflector, and, separately from a transmission antenna on the card reader, a reception antenna is provided by which waves reflected into the atmosphere are received. The reflective property of the dipole antenna of the IC card changes in accordance with a change in the impedance at the antenna. When the transmission antenna of the card reader emits an electromagnetic wave having a specific frequency, the phase or the level of a reflected wave is changed according to the contents of the transmitted data. This change is detected by the reception antenna of the card reader, so that the transmitted data can be specifically identified.
However, in the conventional system described in the above publication a reflected wave emitted into the atmosphere is received by the card reader and the transmitted data is identified. As a result, the conventional system is easily affected by a noise field that occurs in the vicinity of an antenna. Furthermore, two special antennas, a transmission antenna and a reception antenna, must be provided for the card reader, both of which must be highly directional in order to prevent a signal transmitted by the transmission antenna from doubling around and going directly to the reception antenna.
In addition, as is described in the embodiment in the above publication, while the conventional system is adequate for use in the GHz band, its use is difficult at low frequencies in an HF (high frequency) band, such as 14 Mhz, which is actually used for an IC card. To employ the conventional system in the HF band, a dipole antenna having a calculated length of 10 m is required, and it is impossible to mount such an antenna on a compact IC card.
Therefore, one object of the present invention to provide a wireless IC card system that is little affected by a peripheral noise field.